docs: add prototype sphinx extension for online help

goes through the sections in the documents and creates the
OnlineHelpInfo.js file from the explicitly defined section labels which
are used in the js files with 'onlineHelp' variable.

docs/Makefile

@@ -28,7 +28,6 @@ COMPILEDIR := ../target/debug
 SPHINXOPTS    += -t devbuild
 endif
 
-
 # Sphinx internal variables.
 ALLSPHINXOPTS   = -d $(BUILDDIR)/doctrees $(SPHINXOPTS) .
 
@@ -68,6 +67,12 @@ proxmox-backup-manager.1: proxmox-backup-manager/man1.rst  proxmox-backup-manage
 proxmox-backup-proxy.1: proxmox-backup-proxy/man1.rst  proxmox-backup-proxy/description.rst
 	rst2man $< >$@
 
+.PHONY: onlinehelpinfo
+onlinehelpinfo:
+	@echo "Generating OnlineHelpInfo.js..."
+	$(SPHINXBUILD) -b proxmox-scanrefs $(ALLSPHINXOPTS) $(BUILDDIR)/scanrefs
+	@echo "Build finished. OnlineHelpInfo.js is in $(BUILDDIR)/scanrefs."
+
 .PHONY: html
 html: ${GENERATED_SYNOPSIS}
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html